1. Field of the Invention
This invention relates generally to optical correlators, and more particularly to optical correlators for real-time tracking of the position of a moving object. Still more particularly, the invention relates to a joint transform optical correlator for real-time tracking of the position of an image of the interior of the eye and, based upon such tracking directing a surgical laser beam to a precise point of the retina as directed by a surgeon.
2. Description of the Prior Art
Optical pattern recognition has been suggested in various applications because of its parallel processing and high-speed operation capabilities, particularly in real-time applications such as missile guidance, vehicle tracking, and automated lander guidance in aerospace missions. In 1974, VanderLugt in a paper entitled, "Coherent Optical Processing", Proc. IEEE 64, 1300 (1974), described the use of matched spatial filters (MSFs) in performing cloud motion analysis in a sequence of photographs taken from a satellite in half-hour intervals. A new MSF was made for every photograph in order to correlate with the scene in the next picture. Obviously such a method is not a practical real-time technique.
With the recent development of various kinds of spatial light modulators, several real-time tracking methods have been demonstrated. A. D. Gura described a liquid crystal light valve to perform real-time tracking based on a fixed MSF in a VanderLugt correlator in the paper, "Real-time tracking of moving objects by optical correlation," Appl. Opt. 18,172 (1979). T. H. Chao and H. K. Liu described a liquid crystal television spatial light modulator, a dichromated gelatin multifocus hololens and a MSF array for simultaneously tracking multiple objects in their paper, "Real-time optical holographic tracking of multiple objects," Appl. Opt. 28,266 (1989).
Optical tracking using novelty filters and image subtraction have been proposed in which the system detects the difference between sequential image frames and then computes the position of the object in motion. Such proposals are described in the paper by D. Z. Anderson, D. M. Lininger, and J. Feinberg, "Optical tracking novelty filter," Opt Lett. 12,123 (1987) and in the paper by Y. Li, A. Kostregewski, D. H. Kim, and G. Eichman, "Liquid crystal T.V.-based white light optical tracking novelty filter," Appl. Opt. 28, 4861 (1989).
The art of optical pattern recognition described above in general requires matched spatial filters that require intensive computations and individually span only a relatively small range of apperances of an object. A tracking system based on that technology thus suffers from difficulty in adapting to new objects, changes in the inherent appearance of an object, or altered perspective views of an unchanging (but moving) object.
Other methods used digital computation rather than optical filters to perform conversion from one domain to another by means of the Fourier transform. Digital computation is relatively expensive and complicated as compared to optical filtration. Furthermore it suffers from some of the same disadvantages as matched optical filtration because it has to be "taught" the pattern to be recognized on a case by case basis. Such disadvantages have prevented rapid adaptation to changes in the pattern.
Accordingly, a primary general object of the invention is to provide a method for object tracking using a joint transform correlator which does not require the use of a matched spatial filter in the correlation process.
Another general object of the invention is to provide a joint transform correlator for the tracking of a moving object which inherently includes the adaptiveness and high processing speed of an optical system.
A preferred embodiment of the optical joint correlator for image tracking according to the invention is for tracking the movement of the retinal image of an eye surgery patient.
In prior art eye surgery, nystagmic motions of the eye reduce the precision of the surgeon's aim in retinal surgery such as laser-inducted photocoagulation. An eye can be paralyzed against such motions by anesthetic injection but with possible damage to the optic nerve.
Another prior art method to aim the eye surgeon's laser beam is to mount a large contact lens and attached mirrors on the eyeball. As the lens moves with the eyeball, the mirror corrects the motion of the laser beam to compensate for motion of the eyeball. The contact lens method for laser beam pointing to a particular spot on the retina is cumbersome to use and uncomfortable to the patient.
Another prior art eye surgery method is use "Purkinje" images to point a laser beam to a specific spot on the retina of an eye. Purkinje images arise from multiple reflections among surfaces near the front of the eyeball. Such method is indirect in that depends on reflections of structures near the front of the eyeball rather than being a direct image of the retina. Patients with eye disease who require eye surgery frequently also have degradation of the very structures which make the Purkinje images possible.
Accordingly, it is another object of the invention to provide a non-invasive method and apparatus for obtaining a direct image of and for precisely directing a surgeon's laser beam to a point on the retina of an eyeball.
Still another object of this invention is to provide a method and system for automatically adapting to slow changes of appearances from time frame to time frame of a tracked object, specifically a retina of an eye, which may be altered in its appearance during the progress of surgery, which may only be perspectively altered by a surgeon by changing the magnification of an optical viewing system to suit himself during the surgery.